Armature structure for reed switches

ABSTRACT

A sealed switch with an elongated glass housing having pole pieces sealed in both ends includes a uniplanar, rigid armature having a projecting or transversely extending end portion. A flat spring movably mounts said one end on one pole piece with the projection forming a working gap with this pole piece so that the armature has two working gaps and the switch has increased sensitivity. In one switch, a pair of pole pieces are sealed at one end of the housing with the free end of the armature disposed between the pair of pole pieces. In another switch, a single pole piece is provided at each end.

United States Patent [56] References Cited UNITED STATES PATENTS 3,054,873 9/1962 OBrien et al 335/55 3,327,262 6/1967 Bongard et al 335/154 Primary Examiner-Harold Broome Assistant Examiner-R. N. Envall, Jr. Attorney-Mason, Kolehmainen, Rathburn and Wyss ABSTRACT: A sealed switch with an elongated glass housing having pole pieces sealed in both ends includes a uniplanar, rigid annature having a projecting or transversely extending end portion. A flat spring movably mounts said one end on one pole piece with the projection forming a working gap with this pole piece so that the armature has two working gaps and the switch has increased sensitivity. ln one switch, a pair of pole pieces are sealed at one end of the housing with the free end ofthe armature disposed between the pair of pole pieces. ln another switch, a single pole piece is provided at each end.

ARMATURE STRUCTURE FOR REED SWITCHES This invention relates to reed or sealed magnetic switches, and more particularly, to an improved armature structure for both mercury wetted and dry switches.

Reed or sealed magnetic switches have 'been used in a variety of applications in which a magnetic field is used to close or open the switch. Many are now being used in low gain, low voltage circuits and other applications in which the sensitivity of the switch becomes a more important factor. Previous switch designs usually had a single working gap between a moving armature or reed and a pole piece and did not possess as great a sensitivity as desired.

Accordingly, one object of the present invention is to provide a new and improved reed or sealed magnetic switch.

Another object is to provide a new and improved armature and armature mounting structure for a sealed magnetic switch that improves the sensitivity of the switch.

ln accordance with these and many other objects, an embodiment of the present invention comprises a sealed switch having an elongated glass housing in one end of which a magnetic contact structure is sealed, and in the other end of which a magnetic armature supporting structure is sealed. The magnetic amiature disposed within the housing is movably mounted at one end of the supporting structure and has its other end disposed'adjacent the contact structure to form the first working gap in the magnetic circuit of the switch. A flat spring of small cross section mounts the armature on the supporting structure so that a projecting or transversely extending end portion on the armature closely overlies the end of the supporting structure to provide a short, large area second working gap that enables the sensitivity of the switch to be increased. ln one embodiment, the spring mounted end of the armature is L-shaped, and in another embodiment the spring mounted end of the armature is .l-shaped. The armature and the armature mounting structure can be used in mercury wetted or dry contact switches and with the free end of the ar mature disposed adjacent to one pole piece or interposed between a pair of contacts on a set of pole pieces.

Many other objects and advantages of the present invention will become apparent from considering the following detailed description in conjunction with the drawings in which:

FIG. 1 is a sectional view of a mercury switch embodying the present invention;

FIG. 2 is a perspective view of the armature and the magnetic armature supporting structure; and

FIG. 3 is a sectional view of another embodiment of the invention.

Referring now more specifically to FIG. 1 of the drawings, therein is illustrated a sealed mercury wetted switch which is indicated generally as and which embodies the present invention. The sealed switch l0 includes an elongated cylindrical housing l2 formed of glass or other insulating material. ln one end of the housing I2 a pair of similar magnetic terminals or pole pieces l4and 16 are disposed. ln the other end of the housing 12 a single magnetic terminal or pole piece 26 is located. A rigid, magnetic armature 22 is disposed within the housing 12 so that the generally uniplanar free contact end forms the gaps 29A and 29 with pole pieces 14 and I6, respectively. The armature 22 includes a generally uniplanar body portion at the lower end of which is formed a J shaped portion projecting from or extending beyond or transversely to the body portion. The J-shaped end of the armature is movably mounted on the top of the pole piece or armature supporting structure 26 by means of flat spring 23 so that a second working gap 28 is established in the magnetic circuit of the switch. When an appropriate magnetic field is applied to the switch, forces in gaps 29 or 29A and 28 cause the magnetic armature to move so that it makes contact with pole piece 14 or 16. The addition of the second working gap 28 in the magnetic circuit enables the switch to have the desired increased sensitivity.

The inner endsor portions 14A and 16A of the pole pieces 14 and 16, respectively, are sealed within a cavity 18 defined by the housingl2 in positions generally parallel to and spaced from each other. Each of the offset portions 14A and 16A carries a generally spherical contact member 20 that is alternately engaged by the interposed adjacent end of the magnetic armature 22. A pool of mercury 24 is located at the lower end of the cavity 18 so that the armature can be properly wetted. The process of wetting the free contact end of the armature and the end portions of pole pieces 14 and 16 is generally known in the art. The armature 22 and the contacts 20 can provide break-before-make or make-before-break contact action.

The lower end of armature 22 is .l-shaped so that the second working gap 28 is formed with the adjacent generally fiat and transverse end surface of the top portion of the pole piece 26. Magnetic plates 27 and 27A with grooved or other capillary surfaces are welded to the top portion of the pole piece 26 to enhance the capillary movement of the mercury so that the free contact end of the armature can be properly wetted. A flat spring 23 of small cross-sectional area electrically connects and movably mounts the armature 22 to the pole piece 26. The spring 23 is connected to the armature 22 in the vicinity of a point 2l, as by spot welding the spring to an upstanding leg 22A of the .l-shaped end of the armature 22. The

connection of the spring 23 to pole piece 26 is made on thev plate 27A in the vicinity of-a point 21A. This connection also is normally made by spot welding the flat spring 23 to the plate 27A. The general construction of the armature 22 and its movable or pivotal mounting by f'lat spring 23 near the top of pole piece 26 can be more readily seen in FIG. 2 of the drawings.

The magnetic field that traverses the switch 10 must travel through pole piece 26 and armature 22. The magnetic path between armature 22 and pole piece 26 is partly across the working gap 28 which presents a minimum of reluctance, for the gap is of short length and a rather large area. ln addition, the magnetic spring 23 magnetically couples armature 22 to pole piece 26. The spring 23 must be of sufficient length to insure proper flexibility for both the biasing and the swinging or pivoting movement of the armature 22. It is therefore connected at a point 21A, a distance away from the end of pole piece 26. Since the spring 23 is of rather small cross-sectional area, it presents a fairly high reluctance to any magnetic flux travelling through its length from point 21A to point 21. However, by having the spring 23 located so that it closely overlies the pole piece 26, a spacing of minimum length is provided through which the magnetic flux can travel and the path through spring 53 is greatly reduced. `This provides magnetic coupling of the pole piece 26 and the armature 22 by way of the spring 23 with a minimum of reluctance. By maintaining the spacing between the spring 23 and the pole piece 26 at a minimal length, the reluctance can be maintained at a substantially constant low level, and the sensitivity of the switch will not be affected.

The flat spring 23 also resiliently biases the armature 22 to a normal position in which the free end of the armature 22 is spaced from both of the contacts 20 to provide a center-stable switch or sealed contact unit l0. To permit selective operation of the switch l0, oppositely poled magnetic bias is provided for the terminals or pole pieces 14 and 16 in a manner well known in the art, as by the provision of one or more permanent magnets. The bias can be such that the switch is rendered bistable or single-side-stable in dependence on the relative magnitudes of the bias.

Assuming that the switch l0 is to be operated to close a circuit between the armature 22 and the contact 20 on the pole piece 16, a winding (not shown) or other source of a magnetic field is operated to provide a flux field aiding the bias applied between the armature 22 and the pole piece 16, -this field bucking or opposing the bias field applied to the terminal 14. The only field between the armature 22 and the pole piece 16 in the gap therebetween moves the free end of the armature 22 into engagement with the contact 20. ln addition, the field across the gap 28, which field is short and of large area, moves the bight portion of the J-shaped end of the armature 22 somewhat to the right (FIG. l) and downwardly toward the flat top surface of the pole piece 26. This aids the movement of the free end of the armature 22 toward the contact 20 on the pole piece 16. ln the preferred embodiment, the armature 22 does not engage the fiat end surface of the pole piece 26 in the area of the gap 28 to avoid any problem of sticking."

The provision of the second working gap 28 increases the sensitivity of the switch. Although an idealized flux field could be considered to traverse virtually the entire length or an appreciable part of the magnetic circuit of the unit l0, i.e., the pole pieces 14, 16, 26, and the armature 22, there are in practice localized fields of appreciable strength and more limited extent. By the provision of the two working gaps, the gap 28 and the gap at the free end of the armature 22, the operating flux available from the external source is much more efficiently utilized with the resulting increase in sensitivity.

Substantially the reverse operation takes place when the ap plied field aids the bias applied to the pole piece 14. However, the assistance of the gap 28 in moving the armature 22 is less, and this is compensated for when both operate sensitivities are to be the same by using a greater bias on the pole piece 14.

FIG. 3 illustrates another embodiment of the invention which comprises a form A or make contact reed switch 40 that can be either wetted or dry. The elongated cylindrical housing 42 can again be formed of glass or other insulating material into the opposite ends of which two magnetic terminals or pole pieces 44 and 56 are sealed. A generally uniplanar and rigid magnetic armature 52 is disposed within the housing 42 so that its free end portion is adjacent an end portion 44A of the pole piece 44 so that a working gap 59 is established. The generally uniplanar body of the armature 44 is provided at its other end with an L-shaped portion projecting from or extending transversely to the body portion. This L-shaped end is movably mounted on the pole piece 56 by a flat spring 53 so that the L-shaped end closely overlies the top end of pole piece 56 forming a generally transverse, flat surface so that a second working gap Sd is fonned. The addition of this second working gap, which gap is of short length and large area, in the magnetic circuit of the switch enables the switch to have the desired increased sensitivity.

ln addition to movably mounting the armature to the pole piece 56, the small cross-sectional area f'lat spring 53 electrically connects armature 52 to the pole piece 56 and resiliently biases the armature so that when no magnetic field is applied to the switch, the amature will remain spaced from thc contact 50 on the pole piece 44, and the switch will be electrically open. The spring 53 is connected near the Lshaped end of the armature 52 in the vicinity of a point 5l and to the pole piece 56 in the vicinity of a point 51A. These connections are normally made by spot-welding.

A magnetic field traversing the switch 40 substantially travels through the working gap 58 between pole piece 56 and the armature S2. The spring 53 is also of magnetic material so that a second path is established between the armature 52 and the pole piece 56. Since the spring S3 must be of sufficient length to have proper flexibility for biasing and pivotally mounting the armature 52, the reluctance of the path through the spring S3 could be quite high because the spring is of appreciable length and has a small cross-sectional area. However, by having the spring 53 located so that it closely overlies the pole piece S6, a low reluctance path is formed between pole piece 56 and the armature 52 through the spring 53. This low reluctance path and its purpose are described in more detail in conjunction with the switch l0.

A stop member 49 is welded to the armature near its uniplanar end portion. The stop 49 is made so that it is not wettable with mercury, normally being stainless steel, and spherical in shape. The addition of the stop 49 on the armature 52 by engagement with the housing 42 on opening of the switch 40 damps the oscillation of the annature or reed. lf it were not for this damping of the oscillation, there would be a possibility that the armature 52 would make a false contact with the pole piece 44 due to flagging." ln previous designs, the oscillation of reeds was damped by a dimple in the glass housing. However, if such dimples are used with mercury wetted armatures, the armature could become bound to the dimple or glass projection by the adhesion of the mercury at the dimple-armature interface. Since the stop 49 is not wettable with mercury, a dry contact between the housing and the moving armature structure is provided, and any adhesion is eliminated.

ln case the switch is inverted, it is important that any surplus mercury on the armature 52 drain rapidly if the switch 40 is of the mercury wetted type. When the armature 52 is allowed to lie against the glass tubing 42, a narrow capillary spacing is formed that can entrap the mercury and prevent the proper draining of the armature 52, especially before the first operation of the switch after inversion. The addition of the stop 49 prevents the armature 52 from resting against the housing 42 so that no capillary accumulation of mercury occurs, and the draining of the mercury is assisted. Thus, the sensitivity of the first operation of the switch after it is inverted will not differ from the sensitivity of subsequent operations. i

When it is desired to operate the switch 40, a magnetic field of any polarity is applied so that forces working in gap 59 move the armature 52 toward a contact 50 carried on the end portion 44A of pole piece 44. ln addition, magnetic forces within gap 58 pull the transverse part of the L-shaped end of the armature 22 toward the top of the adjacent flat end of the pole piece 56. As can be seen from FIG. 3 of the drawings, when the L-shaped end of armature 52 is pulled toward the top of pole piece 56, it pulls the free contact end of the armature toward contact 50. These physically separated gaps make use of the distributed operating flux fields and increase the sensitivity of the switch 40. When the applied field is removed,

y the resilience of the flat spring 53 restores the switch 40 to the position shown in FIG. 3. The L-shaped end portion of the armature 52 never makes electrical contact with the top of pole piece S6. Therefore the electrical circuit extends only through the flat spring 53.

The use of the reed switch construction 40 also aids in the solution of another problem frequently encountered in making reed switches. ln thetype of reed switch in which the movement of the reeds is obtained by bending a cantilever supported, long reed of uniform cross section adjacent its point of sealing at one end of the glass envelope, the desired degree of flexibility is obtained by severely reducing the thickness of the reed at a point near the seal. Since the deflection of the reed is a function of the cube of its thickness, a very close control of thickness at the point of reduction is necessary. lt is much easier to control the thickness of the uniform strip material of the spring 53, and much closer operatin tolerances are then achieved in the switch 40.

What is claimed and desired to be secured by Letters Patent of the United States is:

l claim:

l. A sealed switch controlled by a magnetic field comprismg:

a housing,

first and second magnetic pole pieces sealed in both ends of thel housing, at least the second of said pole pieces having a generally transverse surface,

a magnetic armature disposed within the housing having a first end portion adjacent said first pole piece so that a first working gap is formed, said armature having a body portion with a second end portion projecting or transversely extending from the body portion,

and spring means coupled to the second pole piece and the second end of the armature and movably mounting the projecting portion of the armature in a spaced and generally parallel relation to the transverse surface of the second pole piece to define a second working gap.

2. The switch set forth in claim 1 including a mercury wetted contact means on trie first pole piece and the first end portion of the magnetic arm ature.

3. The sealed switch set forth in claim l in which:

the armature is generally uniplanar and said second end of the armature is generally L-shaped,

and the spring means is generally uniplanar with one end connected to the armature immediately adjacent the L- shaped end of the armature.

4. The sealed switch set forth in claim 3 in which the other end of the uniplanar spring means is connected to the second pole piece at a position spaced away from said transverse surface.

5. The sealed switch set forth in claim 1 in which:

the amature is generally uniplanar and said second end of the armature is generally .l-shaped with a leg extending generally parallel to and spaced from the armature,

and the spring means is generally uniplanar with one end connected to the leg of the J-shaped portion of the armature.

6. The sealed switch set forth in claim 5 in which:

the other end of the spring means is connected to the second pole piece spaced away from said transverse surface.

7. A sealed switch as set forth in claim 1 including:

a stop member attached to the magnetic armature near said first end portion and so located on the armature relative to the housing and armature that in movement of the armature away from the first pole piece, the stop member engages the housing to prevent contact between the armature and the housing. l

8. A switch as set forth in claim l in which:

the spring means in magnetic, has one end connected to the second end of the armature closely adjacent the second end of the armature, has its other end connected to the second pole piece at a point spaced away from the second working gap, and closely overlies the second magnetic pole piece between its other end and the second working gap so that a spacing of fairly constant minimal length is formed between the second pole piece and the spring.

9. A sealed mercury switch controlled by a lmagnetic field comprising:

a housing,

first and second magnetic pole pieces sealed in one end of the housing,v

a rigid magnetic armature disposed within the housing and having a first end portion coated with mercury on both sides and interposed between the first and second pole pieces so that a gap is fonned between the first and second pole pieces and said first end portion of the armature, said armature having a body portion with a structure projecting from the body portion to form a transverse surface at its second end,

a third magnetic pole piece sealed in the other end of the housing and having a transverse surface, y

and spring means coupling the armature and the third pole piece and movably mounting the armature adjacent the third magnetic pole piece with the transverse surfaces of the third pole piece and the armature disposed in a spaced and generally parallel relation to form a working gap.

l0. The sealed switch set forth in claim 9 in which:

the body portion of the armature is generally uniplanar and the projecting structure at the second end of the armature forms a J-shaped portion with a legextending generally parallel to and spaced from the armature,

and the spring means is generally uniplanar with one end connected to the leg of the J-shaped portion of the armature.

1l. The sealed switch set forth in claim l0 in which the other end of the spring means is connected to the second pole piece spaced away from said transverse surface.

12. A switch as set forth in claim 9 in which:

the spring means is magnetic, is connected to the third pole piece spaced from said working gap, and closely overlies the second magnetic pole piece between the point of connection to the third pole piece and said working gap so that a spacing of fairly constant minimal length is fonned between.

13. A sealed switch controlled by a magnetic field comprising:

an elongated housing having an axis along its length,

k first and second magnetic pole pieces sealed in opposite ends of the housing and aligned on the said axis, at least the second of thc pole pieces having a generally transverse surface,

a rigid magnetic armature so disposed within the housing that its axis is aligned with or slightly displaced from the axis of the housing and having a first end portion adjacent said first pole piece so that a first working gap is formed, the armature having a body portion from which a structure projects at the second end of the armature to form a transverse surface,

and spring means coupling the second pole piece to the other end of the armature and movably mounting the armature adjacent the second pole piece with the transverse surfaces of the second pole piece and the armature disposed in a spaced and generally parallel relation to define a second working gap.

14. The sealed switch set forth in claim 13 including a mercury wetted contact means on the first pole piece and the first end portion of the magnetic armature.

l5. A sealed switch as set forth in claim 13 including a stop member attached to the magnetic armature near said first end portion and so locatedon the armature relative to the housing and armature that in movement of the armature away from the first pole piece, the stop member engages the housing to prevent contact between the armature and the housing.

16. A switch as set forth in claim 13 in which:

the spring'means is magnetic and closely overlies the second magnetic polepiece so that a spacing of fairly constant minimal length is formed between the pole piece and the spring.

17. A sealed mercury switch controlled by a magnetic field comprising:

a housing,

a first magnetic pole piece sealed in one end of the housing,

a magnetic armature disposed within the housing having a free end portion,

a mercury wetted contact means on the first pole piece and said free end portion,

a stop member attached to the armature near said free end portion and so located on the armature relative to the housing and the armature that in movement of the armature away from the first pole piece, the stop member engages the housing to prevent contact between the armature and the housing, the stop member not being wettable with mercury,

and a second magnetic pole piece sealed in the other end of the housing to which the other end of the armature is pivotally mounted.

18. The switch set forth in claim 17 in which the stop member is spherical in shape.

19. A sealed mercury switch controlled by a magnetic field comprising:

a housing,

first and second magnetic structures sealed in the housing at points spaced from each other,

said first magnetic structure including a movably mounted magnetic armature disposed within the housing and having a free end portion movable into and out of engagement with the second magnetic structure,

a mercury wetted contact means on the free end portion of the armature,

and a stop member attached to the armature near said end portion and so located on'the armature relative to the housing and the` armature that on movement of the armature away from the second magnetic structure, the stop member engages the housing to prevent contact between the armature and the housing, the stop member not being wettable with mercury.

20. A switch as set forth in claim 19 in which the stop member is of spherical shape.

21. A sealed switch controlled by a magnetic field comprising:

a housing,

first and second magnetic structures sealed in the housing at points spaced from each other, said first magnetic structure including a magnetic pole piece having a generally transverse surface and a magnetic armature disposed within the housing having a free end portion movable into and out of .engagement with the second magnetic structure, the other end of the armature having a projecting transverse surface, f

and a magnetic spring means movably mounting the said armature adjacent said pole piece with the transverse surfaces of the pole'piece and the armature disposed in a A 

1. A sealed switch controlled by a magnetic field comprising: a housing, first and second magnetic pole pieces sealed in both ends of the housing, at least the second of said pole pieces having a generally transverse surface, a magnetic armature disposed within the housing having a first end portion adjacent said first pole piece so that a first working gap is formed, said armature having a body portion with a second end portion projecting or transversely extending from the body portion, and spring means coupled to the second pole piece and the second end of the armature and movably mounting the projecting portion of the armature in a spaced and generally parallel relation to the transverse surface of the second pole piece to define a second working gap.
 2. The switch set forth in claim 1 including a mercury wetted contact means on the first pole piece and the first end portion of the magnetic armature.
 3. The sealed switch set forth in claim 1 in which: the armature is generally uniplanar and said second end of the armature is generally L-shaped, and the spring means is generally uniplanar with one end connected to the armature immediately adjacent the L-shaped end of the armature.
 4. The sealed switch set forth in claim 3 in which the other end of the uniplanar spring means is connected to the second pole piece at a position spaced away from said transverse surface.
 5. The sealed switch set forth in claim 1 in which: the armature is generally uniplanar and said second end of the armature is generally J-shaped with a leg extending generally parallel to and spaced from the armature, and the spring means is generally uniplanar with one end connected to the leg of the J-shaped portion of the armature.
 6. The sealed switch set forth in claim 5 in which: the other end of the spring means is connected to the second pole piece spaced away from said transverse surface.
 7. A sealed switch as set forth in claim 1 including: a stop member attached to the magnetic armature near said first end portion and so located on the armature relative to the housing and armature that in movement of the armature away from the first pole piece, the stop member engages the housing to prevent contact between the armature and the housing.
 8. A switch as set forth in claim 1 in which: the spring means in magnetic, has one end connected to the second end of the armature closely adjacent the second end of the armature, has its other end connected to the second pole piece at a point spaced away from the second working gap, and closely overlies the second magnetic pole piece between its other end and the second working gap so that a spacing of fairly constant minimal length is formed between the second pole piece and the spring.
 9. A sealed mercury switch controlled by a magnetic field comprising: a housing, first and second magnetic pole pieces sealEd in one end of the housing, a rigid magnetic armature disposed within the housing and having a first end portion coated with mercury on both sides and interposed between the first and second pole pieces so that a gap is formed between the first and second pole pieces and said first end portion of the armature, said armature having a body portion with a structure projecting from the body portion to form a transverse surface at its second end, a third magnetic pole piece sealed in the other end of the housing and having a transverse surface, and spring means coupling the armature and the third pole piece and movably mounting the armature adjacent the third magnetic pole piece with the transverse surfaces of the third pole piece and the armature disposed in a spaced and generally parallel relation to form a working gap.
 10. The sealed switch set forth in claim 9 in which: the body portion of the armature is generally uniplanar and the projecting structure at the second end of the armature forms a J-shaped portion with a leg extending generally parallel to and spaced from the armature, and the spring means is generally uniplanar with one end connected to the leg of the J-shaped portion of the armature.
 11. The sealed switch set forth in claim 10 in which the other end of the spring means is connected to the second pole piece spaced away from said transverse surface.
 12. A switch as set forth in claim 9 in which: the spring means is magnetic, is connected to the third pole piece spaced from said working gap, and closely overlies the second magnetic pole piece between the point of connection to the third pole piece and said working gap so that a spacing of fairly constant minimal length is formed between.
 13. A sealed switch controlled by a magnetic field comprising: an elongated housing having an axis along its length, first and second magnetic pole pieces sealed in opposite ends of the housing and aligned on the said axis, at least the second of the pole pieces having a generally transverse surface, a rigid magnetic armature so disposed within the housing that its axis is aligned with or slightly displaced from the axis of the housing and having a first end portion adjacent said first pole piece so that a first working gap is formed, the armature having a body portion from which a structure projects at the second end of the armature to form a transverse surface, and spring means coupling the second pole piece to the other end of the armature and movably mounting the armature adjacent the second pole piece with the transverse surfaces of the second pole piece and the armature disposed in a spaced and generally parallel relation to define a second working gap.
 14. The sealed switch set forth in claim 13 including a mercury wetted contact means on the first pole piece and the first end portion of the magnetic armature.
 15. A sealed switch as set forth in claim 13 including a stop member attached to the magnetic armature near said first end portion and so located on the armature relative to the housing and armature that in movement of the armature away from the first pole piece, the stop member engages the housing to prevent contact between the armature and the housing.
 16. A switch as set forth in claim 13 in which: the spring means is magnetic and closely overlies the second magnetic pole piece so that a spacing of fairly constant minimal length is formed between the pole piece and the spring.
 17. A sealed mercury switch controlled by a magnetic field comprising: a housing, a first magnetic pole piece sealed in one end of the housing, a magnetic armature disposed within the housing having a free end portion, a mercury wetted contact means on the first pole piece and said free end portion, a stop member attached to the armature near said free end portion and so located on the armature relative to the housing and the armature that in movement of the armature away from the fiRst pole piece, the stop member engages the housing to prevent contact between the armature and the housing, the stop member not being wettable with mercury, and a second magnetic pole piece sealed in the other end of the housing to which the other end of the armature is pivotally mounted.
 18. The switch set forth in claim 17 in which the stop member is spherical in shape.
 19. A sealed mercury switch controlled by a magnetic field comprising: a housing, first and second magnetic structures sealed in the housing at points spaced from each other, said first magnetic structure including a movably mounted magnetic armature disposed within the housing and having a free end portion movable into and out of engagement with the second magnetic structure, a mercury wetted contact means on the free end portion of the armature, and a stop member attached to the armature near said end portion and so located on the armature relative to the housing and the armature that on movement of the armature away from the second magnetic structure, the stop member engages the housing to prevent contact between the armature and the housing, the stop member not being wettable with mercury.
 20. A switch as set forth in claim 19 in which the stop member is of spherical shape.
 21. A sealed switch controlled by a magnetic field comprising: a housing, first and second magnetic structures sealed in the housing at points spaced from each other, said first magnetic structure including a magnetic pole piece having a generally transverse surface and a magnetic armature disposed within the housing having a free end portion movable into and out of engagement with the second magnetic structure, the other end of the armature having a projecting transverse surface, and a magnetic spring means movably mounting the said armature adjacent said pole piece with the transverse surfaces of the pole piece and the armature disposed in a spaced and generally parallel relation to define a working gap, said spring means coupled to the armature near the transverse surface of the armature and to the pole piece a distance spaced from the transverse surface of the pole piece so that the spring means closely overlies the pole piece and a spacing of fairly constant minimal length is formed between the pole piece and the spring. 